Bran is defined as the seed coat of cereal grains such as wheat, barley, rye, triticale, oat or rice. Anatomically, bran comprises the outer layers of the seed, known as the pericarp-testa and an inner layer known as the aleurone layer, which is often classified as the outermost layer of the endosperm. However, from the practical point of view cereal bran is herein defined as the remaining material after the conventional milling or polishing of cereal grains and contains primarily pericarp-testa and aleurone layer components, along with the cereal germ and residual parts of the endosperm. The relative amounts of each component will depend upon the type of cereal and milling technique applied.
Within this definition, bran therefore contains all of the pericarp-testa components, the aleurone layer, the germ components including germ proteins and oils, along with a residual amount of endosperm starch, gluten and pentosans.
U.S. Pat. No. 4,361,651 describes a process for making fermentable sugars and high protein products from grain, mainly maize. In this method, grain is steeped for 10-30 h, prior to milling and separation of the germ component, saccharification of carbohydrates (mainly starch), and separation of fibre. The yield of starch is maximised for fermentation to alcohol. Within the described process there is no specific fractionation of the bran component, separation of protein types or consideration of the germ component.
U.S. Pat. No. 5,312,636 discloses a process for fractionating crop into industrial raw material. This is focused on oat grain and incorporates bran fractionation procedures that involve the extraction of more hydrophobic components such as lipids in polar organic solvents prior to the alkaline extraction of residual bran to produce beta-glucan, protein and degummed fibres. The use of the organic solvent is a key step in the process and hydrolysing enzymes are not utilised during the fractionation procedure.
Two related US patents (U.S. Pat. Nos. 4,171,383 and 4,171,384) disclose dry and wet milling procedures for refining whole wheat grain. U.S. Pat. No. 4,171,383 focuses on wet milling of the whole kernel. The bran produced is mixed with a separated (mainly) endosperm protein fraction to produce animal feed. U.S. Pat. No. 4,171,384 describes dry milling of the whole kernel to produce an endosperm fraction, a germ fraction and a bran fraction. The endosperm fraction is then subjected to wet milling and separation of starch-rich and protein-rich fractions. The protein rich fraction is added to the bran to produce an animal feed. There is no description of a specific wet fractionation of the bran itself within either patent.
Patent application WO 99/11672 discloses a process that uses selective enzymes, such as acetyl xylan esterase and ferulic acid esterase, to both facilitate the removal of hemicellulose from various plant materials and alter its degree of phenolic ester substitution. Despite the fact that functional hemicellulose with high solubility and gelling strength can be produced yields are rather low. In fact, the inventors reported a 3 and 6% yield of arabinoxylan ferulate (hemicellulose), when wheat bran was treated with acetyl xylan esterase for 90 and 180 min, respectively. Furthermore, the invention does not make any reference to the use of xylanases, or its combination with wet milling in order to overcome the low yields reported.
U.S. Pat. No. 5,308,618 discloses a process to extract soluble dietary fibre hemicelluloses from wheat bran by applying a heat pre-treatment in aqueous solution. Further processing such as filtration, salting out, dialysis, ultrafiltration, reverse osmosis, gel filtration and precipitation in order to remove contaminants from the hemicellulose fraction, follows this. The inventors make no claims with regards to the use of enzymes and production of products streams other than hemicellulose. Furthermore, the invention highlights the need of run costly procedures to remove contaminants, which were once present in the original wheat bran. The bran is extracted at high temperatures and pressures in water (180-200° C.), producing a glucose rich dietary fibre component in the water phase. The process specifically targets the production of dietary fibre and is not really/strictly a fractionation procedure in that other products is largely ignored.
U.S. Pat. No. 3,879,373 discloses a process to extract hemicelluloses from wheat bran by applying alkali treatment to dissolve hemicelluloses and other bran components followed by ethanol extraction to separate the hemicelluloses. Alkali (sodium hydroxide) extraction of hemicellulose has also been disclosed in U.S. Pat. No. 5,174,998 as an intermediate step to produce controlled-release compositions containing the said alkali-extracted hemicellulose and an active substance. Similar alkali-extraction procedure is disclosed in U.S. Pat. No. 4,927,649 to produce hemicellulose, which is then used in coating compositions containing insoluble dietary fibre.
WO 00/04053 patent application describes a chemical process using alkaline peroxide treatment to produce high yields of light coloured gelling hemicelluloses from products derived from flour, husk or bran. Another chemical extraction process of hemicellulose from wheat bran has been disclosed in WO98/31713 patent application, whereby the inventors combine a washing procedure to remove the starch fraction followed by an alkaline treatment with sodium hydroxide to extract the hemicellulose from the starch-free raw material.
It appears from above-described prior art on alkali extraction of hemicellulose that this is an old, proven and effective way to yield high quantities of soluble hemicellulose with interesting functionalities such as gelling, dietary fibre and as an inert material for controlled-release compositions. The drawback of such technology is the associated problems of utilising chemicals. Firstly, chemicals eventually become contaminants in various product streams, and therefore require additional purification. This normally has significant cost implications. Secondly, innovative industrial processes based on chemical extraction are not always attractive from the marketing point of view, particularly in food applications.
Production of insoluble dietary fibres from oats is disclosed in U.S. Pat. No. 5,023,103, which describes a chemical procedure (alkali and bleaching treatment) for the production of insoluble dietary fibre with high water holding capacity and non-gritty mouth feel. A water holding capacity of 6.9 g watering oat fibre has been reported.
Other references have disclosed processes for the extraction of proteins from cereal brans. U.S. Pat. No. 4,746,073 discloses a physical process to separate aleurone cell particles and pericarp-testa particles from commercial wheat bran. The process consists of milling the bran particles to a specific particle size distribution, electrostatically charge the said particles and then pass the said charged particles through a magnetic field, which separates aleurone from pericarp-testa particles. The separation is achieved by hammer-milling the bran and then subjecting the resultant particles to a physical separation regime achieve the separation. No aqueous wet processing is employed during the fractionation procedure described therein.
This is a rather different concept from the current invention, which is based upon the use of enzymes and aqueous wet milling.
Waszczynskyj et al. (1981) have proven that protein extraction rate of alkali-treated full fat wheat bran can be increased from 30% up to 38.5% when it is preceded by polysaccharidase treatment. The above-mentioned figures are significantly lower than those described in the present invention whereby up to 60% protein extraction rates were achieved without using alkali treatment. Furthermore, U.S. Pat. No. 5,622,738 discloses a method to extract soluble hemicelluloses, for use as a source of dietary fibre, from various fibrous materials including cereal brans using alkali digestion followed by xylanase treatment. As in other prior art, Waszczynskyj et al made use of alkali digestion to improve extraction rates. Additionally, the residence time for the enzymatic treatment was rather long (3 to 96 h), which makes the process not very attractive from a production cost point of view.
WO 01/60180 relates to process for separating oil from rice bran, whereby bran having a suitable particle size in a slurry is subjected to an enzymatic treatment, and subjecting the enzymatically treated slurry for a separation to recover an oil phase for further isolation of specific lipids. The process is carried out under alkaline conditions for a considerable time period, normally 15 hours. As no degradation of starch present, about 15% of the ingoing bran, takes place, any end product will be heavily contaminated with starch.
It is clear that the above-mentioned prior art has not succeeded to arrive at a process of cereal bran fractionation, which is both chemical-free and yields different food-grade fractions and simultaneously yield aleurone proteins, oligosaccharides and hemicelluloses, and yet produce insoluble dietary fibre from previously cleaned cereal bran, i.e. substantially free of soluble components, using xylanases and/or beta-glucanases in combination with wet milling.